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Rainwater Harvesting

INTRODUCTION 

Water as we all know is very essential for our day to day living and that it is becoming an increasingly scare resource. No living being, including plants can survive without water. Yet, those of us who have grown up in cities tend to take water for granted. We have not given water and the need to conserve it the importance that it richly deserves. How many of us indeed are aware that the ultimate source of water we get is the rain that falls during the monsoons?            We get our daily requirement of water from two sources: 1) Water conserved during monsoon – treated and supplied by the water Board and 2) the water drawn from the soil below our homes, known as ground water.           

Till about thirty years back, the areas around our homes and offices used to be unpaved and the rain falling on these areas would percolate into the soil and remain there for being drawn through shallow open wells. With the proliferation of flat complexes, not only have these areas been paved and percolation of rainwater into the soil almost totally stopped, the quantity of water drawn from the soil below has increased manifold. Consequently, open wells and not-so-deep bore wells started drying up. The reason is that no sincere attempt is made to replenish the ground water table with rainwater during the monsoon.           

As individuals, groups and communities, let us all wake up before it is too late and not only understand what rainwater harvesting is all about but also implement measures to harvest rainwater in our houses and flat complexes and put it into the soil for our subsequent use.  

RAINWATER HARVESTING

Catching rainwater when and where it falls for use during nonmonsoon months is called rainwater harvesting.  This can be done in two ways : as surface water by diverting the rainwater into tanks , ponds etc. or as ground water by ingesting it into the soil.            

In cities, due to shrinking of open spaces, rainwater can be harvested only as ground water.  

NEED FOR RAINWATER HARVESTING            

Specially for areas around Alibag and other Cities receiving an average annual rainfall of around 129 cm during a period of two to three months on an average of 300 hours throughout the year.  These characteristics of our rainfall force us not only to conserve large quantity of rainwater during these months but also to store wherever it rains, as ground water.  Failure to do so results in either flooding of low lying areas or wastage by means of run off into the sea.            

What is presently happening in cities like Mumbai is that the areas around houses and flat complexes are paved indiscriminately. As a result all the rainwater runs off into the road and floods the area making life miserable. It is quite possible to put all this water into the soil below with a little effort and expenditure so that the precious rainwater is not lost but becomes available for our use subsequently.  

BENEFITS OF RAINWATER HARVESTING

·  Rainwater harvesting replenishes the ground water table and enables our dug wells and bore wells to yield in a sustained manner.

·  If the ground water happens to be brackish, harvesting will reduce the salinity. In areas where the water becomes yellow due to presence of iron salts, rainwater harvesting will progressively leach out these salts leading to clean water availability in the long run.

· Flooding of low lying areas and roads can be avoided to a large extent, since rainwater that is not harvested both within the house as well as outside is mainly responsible for flooding.  

METHODS OF RAINWATER HARVESTING            

Methods of harvesting rainwater are different for different areas and therefore it is site and soil specific.  It can mean anything from collecting it in a vessel to ingesting into the soil. 

Traditional rainwater harvesting, which is still prevalent in rural areas, was done in surface storage bodies like irrigation tanks ( known as eri in tamil ), lakes, ponds etc. In urban areas, due to shrinking of open spaces,  rainwater will have to necessarily be harvested as ground water.  Hence, harvesting in such places will depend very much on the nature of the soil viz. clayey, sandy, rocky etc.            

In houses and flat complexes, rainwater falls on both the terrace (rooftop) as well as on the area all around ( driveway).  It is this water, which will have to be led into the ground wherever possible.  

ROOFTOP HARVESTING            

Rooftop rainwater is of a good quality as it falls on clean terraces and is brought down by the drain pipes called rooftop pipes. I)          Direct at least one or more of these pipes located close to the existing sump ( meant for receiving metro water) into it through a filter.

Note : 1.  These filters , which are used only to remove suspended impurities in rooftop water, can be masonry tanks measuring 2.5ft. x 2.5 ft. and about 3 ft. deep. One third of the filter alone should be filled with coarse river sand sandwiched between tow layers of blue metal / pebbles. A  nylon mesh should be spread in between the bottom layer of pebbles and sand. The remaining two thirds should be left unfilled for smooth flow of water.  A PVC drum can also be used for this purpose.

Note : 2.   In cases of diversion of rainwater into sumps, the filter will have  to be located above the ground level and there should be enough space available for it.

Note :3.  Diversion to sumps is recommended only in places where there is no or very good little municipal supply and where water is purchased even on rainy days.  II)        Any overflow from the sump can be led into an open well, if any, within the premises.  Pipes not directed to the sump can also be led into the well (fig.1)Fig.1  RAINWATER HARVESTING TO SUMP AND OPEN WELL USING FILTER III)       In the absence of an open well, a percolation/ recharge well could be dug (fig.2) and the same can be made use of to put the rooftop water into it.  

FIG.2 PERCOLATION /RECHARGE WELL 

These are constructed using cement rings readily available in the market.  The diameter of these rings range from 2.5 ft. to 6.0 ft depending on the volume of water that is likely to be ingested into each one of them.  The depth to which these wells are dug depends on the nature of the soil.  They are left unfilled and are covered with RCC slabs of suitable thickness to facilitate vehicular movement on them.  IV)       In houses/flat complexes where there is not enough space around the built up area to dig a recharge well,  a percolation/recharge pit could be made (fig.3) for the purpose of putting rooftop water into it.  

FIG.3 PERCOLATION / RECHARGE BORE PIT V)        In areas where the soil is likely to be clayey upto say, 15ft. or more, it is advisable to go in for a percolation / recharge well cum bore pit.

(fig. 4. ) FIG 4. PERCOLATION/RECHARGE WELL CUM BORE P/T 

A Percolation/recharge pit is a hand bore made in the soil with the help of an augur and filled up with pebbles and river sand on top. The depth of these pits will be anywhere between 4 and 8 metres depending on the nature of the soil.  The pit has to be dug to a depth till a reasonably sandy stratum is reached.  The diameter of the pits will be 25 cms.  ( 10 ins).  A square/circular collection chamber with silt arrestor is provided at the top.  Instead of filling up with pebbles, which is done only to prevent caving in of the bore, a PVC pipe 4 or 6 in. diameter can also be inserted for the entire depth.  In areas where the soil is likely to be clayey upto, say, 15 ft and above, it is advisable to go in for a percolation / recharge will upto 10 to 15 ft. and a hand bore pit within this well upto a depth of another 10 to 15 ft. from the bottom.  A PVC pipe of 4 or 6 in. diameter is inserted into the bore for the entire length and slots are made on the casing and a nylon mesh is wound around it.  The outer well is filled with a layer  of pebbles and coarse river sand.  These precautions are taken to prevent  silt from entering the bore pit.  The same method can be used for charging an existing  live or defunct bore well.  

DRIVEWAY RUNOFF HARVESTING            

There is a general feeling among people that only rooftop water is fit for harvesting and not the driveway runoff.  This is so because surface runoff both at the micro and macro levels appears to be dirty and hence though unifit for harvesting . It should be borne in mind that it is only suspended impurities and can still be put into a recharge well, where the soil will be able to filter it.  It is true that surface runoff water should not be led into a sump for immediate use or to a source well.             

In a large number of houses/flat complexes, the driveway are ( all around the built-up area ) will be as much or even more than the rooftop area.  Rainwater falling on this area will be quite large and in addition, a sizeable  quantity of rooftop  water will also contribute to this, which eventually runs off to the street through the gate(s).  Hence , harvesting driveway runoff  in such places becomes very important.  This should be harvested by intercepting it with the help of a shallow gutter (covered with a perforated RCC slab) near the gate(s) and directed to a recharge well (s). Such  driveway runoff should not be led into a recharge pit since the runoff will contain large amounts of silt and will result in clogging . A cheaper alternative to the gutter  will be a bump (speed breaker). 

COST OF RAINWATER HARVESTING            

The cost of implementation of  rainwater harvesting systems in a house/flat complex will depend on the size of the premises, number and location of the rooftop pipes,  nature of the soil as well as on the availability of an open well.  If the site has an open well there will be no need for creating a recharge well and harvesting will therefore be cheaper.  Secondly, digging a recharge will be in a place where the soil is reasonably sandy will be less expensive than in a place where the soil is clayey.  The cost will vary from Rs. 3000/- for an independent  house Rs. 30000/- for a flat complex, where the cost is shared by the number of flats and will therefore be quite small. Once put in place, the harvesting structures do not require any serous maintenance and hence there is no recurring expenditure.  

RAINWATER HARVESTING IN A FLAT COMPLEX  

FIG.7 ROOFTOP AND DRIVEWAY RUNOFF HARVESTING  

RAINWATER HARVESTING IN INDIVIDUAL HOUSES

FIG.5 RAINWATER HARVESTING USING OPEN WELL AND RECHARGE WELL FIG.

6 RAINWATER HARVESTING USING SUMP AND OPEN WELL 

RAINWATER HARVESTING  PRECAUTIONS, SUGGESTIONS AND TIPS

1. If there is already an open well within the premises, diverting some or all the rooftop water into it should be given priority , particularly in areas where the soil is clayey or rocky.  This will save on the cost of creating  a recharge well.

2. If the well is either dry or contains water on non-potable quality, avoid using a filter.  It is essential only if the well water is suitable  for drinking  and cooking .

3. Avoid using a filter unless it is really essential, since they are not scientifically designed and leads to overflow and wastage of water from the filter itself.  

4. A recharge well (fig.2) though it is more expensive than a recharge pit (fig.3) is definitely a better rainwater harvesting structure.  The former will take in more water and can also be desalted easily, whereas a recharge pit will get clogged very good often and will have to be abandoned.

5. No filtering is required for leading water into the soil for recharge purposes and therefore a recharge well should not be filled with pebbles/blue metal/brickbats .  Ingested water in a recharge well moves to the source (open or bore well) through the soil and in the process gets filtered of even bacteria.

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